Of course mpg , rpm and accellaration will change when going from 3.42 to 4.10's .......All depends on tire size ......which will dictate rpm, city mileage will be a good or maybe even better .......my case 3.42 with 31 tires when I went to 3.90's Highway stayed the same BUT falls off if I cruise at 75 plus ...rpm past 2300 rpm mileage suffers ......so go online and find a good rpm calc check out all the scenarios best mileage for me is at 2100-2200 rpm at cruise If you dont tow then gear change only makes sense if you adding taller tires or want better track performance......... if you gpoing fpor mileage put the stock t-stat back in

 

are those the yukon 3.90s? How did you acceleration change from 3.42s to 3.90?

 

just asking what does NA stand for?

 

naturally aspirated - you are running the engine off air at atmospheric pressure